Capture all displays inside the viewport (macOS)

Capture all displays inside the viewport (macOS)

It was only possible to capture either the current, or all displays
previously. This could become an issue if you want to show a vnc session
over only a selected set of displays, because then we only want to
capture those displays.

The solution in the commit was chosen because it handles all cases - by
looking at what monitors are enclosed by the viewport the implementation
is independent on any configuration, but instead captures the monitors
actually being used.